While motor vehicle manufacturers continue to seek improved fuel economy through engine design; new approaches in formulating engine oils have played an important role in improving fuel economy and have resulted in improved emission characteristics of motor vehicles. Lubricant optimization is especially preferred over engine hardware changes, due to its comparative lower cost per unit in fuel efficiency and possibility for backward compatibility with older engines. Therefore, formulators are under continued pressure to develop engine oils and additive packages which take advantage of new performance basestocks and additive blends which demonstrate better fuel efficiency, oxidative stability, volatility, and improved viscosity index (to name a few characteristics) over conventional formulations. To improve fuel efficiency, there has been a drive to use lower viscosity engine oils, which often requires new additive package formulations. High on the list of requirements for these new formulated engine oil specifications are those employing components which improve the frictional properties of the lubricating oil composition. In this case, the additive system design is the crucial factor and close attention must be focused on the additive/additive and additive/base fluid interactions.
Engine oil acts as a lubricant between moving engine parts at various conditions of load, speed and temperature. Hence, the various engine components experience different combinations of boundary layer, mixed and (elasto) hydrodynamic regimes of lubrication; with the largest frictional losses at piston liner/piston ring interface and a smaller part by the bearing and valve train. To reduce the energy losses due to friction of the various parts and to prevent engine wear, additives are incorporated into the engine oil such as friction modifiers, anti-wear agents, and antioxidants; the latter of which tend to lengthen the effect of the afore mentioned additives. Also to reduce the hydrodynamic friction in the piston/cylinder, the viscosity of engine oils has been lowered which has increased the dependence of friction modifiers to offset the new boundary layer regime. Hence, a vast amount of effort has focused on the interaction of oil viscosity with various friction modifiers to improve fuel economy.
Friction modifiers have been around for several years for application in limited slip gear oils, automatic transmission fluids, slideway lubricants and multipurpose tractor fluids. With the desire for increased fuel economy, friction modifiers have been added to automotive crankcase lubricants and several are known in the art. They generally operate at boundary layer conditions at temperatures where anti-wear and extreme pressure additives are not yet reactive by forming a thin mono-molecular layers of physically adsorbed polar oil-soluble products or reaction layers which exhibit a significantly lower friction compared to typical anti-wear or extreme pressure agents. However, under more severe conditions and in mixed lubrication regime these friction modifiers are added with an anti-wear or extreme pressure agent. The most common type is a zinc dithiophosphate (ZnDTP or ZDDP), which, due to emissions considerations, has been reduced in concentration in many current formulations.
Organo-molybdenum compounds are among the most common metal-containing friction modifiers. Typical organo-molybdenum compounds include molybdenum dithiocarbamates (MoDTC), molybdenum dithiophosphates (MoDTP), molybdenum amines, molybdenum alcoholates, and molybdenum alcohol-amides. WO-A-98/26030, WO-A-99/31113, WO-A-99/47629 and WO-A-99/66013 describe tri-nuclear molybdenum compounds for use in lubricating oil compositions. However, the trend towards low-ash lubricating oil compositions has resulted in an increased drive to achieve low friction and improved fuel economy using ashless (organic) friction modifiers.
Ashless (organic) friction modifiers typically comprise esters of fatty acids and polyhydric alcohols, fatty acid amides, amines derived from fatty acids and organic dithiocarbamate or dithiophosphate compounds. Further improvements in lubricant performance characteristics have been achieved through the use of synergistic behaviours of particular combinations of lubricant additives. While numerous combinations of friction modifiers have been made there remains a need to find improvements and synergies between friction modifiers to improve frictional losses and to potentially improve fuel economy and provide cost benefits.
EP-A-1367116, EP-A-0799883, EP-A-0747464, U.S. Pat. No. 3,933,659 and EP-A-335701 disclose lubricating oil compositions comprising various combinations of ashless friction modifiers. Glycerol monooleate (GMO) is well known to function as a friction modifier in lubricant compositions for engines. See, e.g., U.S. Pat. Nos. 5,885,942; 5,866,520; 5,114,603; 4,957,651; and 4,683,069. For example, U.S. Pat. Nos. 5,114,603 and 4,683,069 describe lubricating oil compositions comprising mixtures of glycerol monooleate and glycerol dioleate in combination with other additives which were added for their conventional purpose.
U.S. Pat. No. 5,286,394 discloses a friction-reducing lubricating oil composition and a method for reducing the fuel consumption of an internal combustion engine. The lubricating oil composition disclosed therein comprises a major amount of an oil having lubricating viscosity and a minor amount of a friction-modifying, polar and surface active organic compound selected from a long list of compounds including mono- and higher esters of polyols and aliphatic amides. Glycerol monooleate and oleamide (i.e. oleylamide) are mentioned as examples of such compounds.